The Somali government says al-Shabab fighters have attacked troops in northern Mogadishu, continuing a string of attacks in the capital after a period of relative calm.

Officials say the al-Qaida-linked group mounted an offensive on government positions Thursday in the northern Huriwa and Karan districts.

Both sides claimed to have caused enemy casualties, although their reports could not be independently confirmed.

Al-Shabab withdrew its fighters from the capital in August after the government launched an offensive with the support of African Union troops. The Islamist group said the move was "tactical" and vowed to return.

Since late November, there have been 15 bomb attacks in Mogadishu, and the government has blamed them on al-Shabab.

In an interview with VOA Somali Service on Thursday, a former official with Somalia's National Security Agency said al-Shabab appears to have adopted guerilla tactics to destabilize Mogadishu.

Colonel Abdulahi Ali Maow also said al-Shabab appears to have created a unit of more sophisticated fighters, some of whom may have infiltrated pro-government forces.